Existing clot retrieval devices are based on the concept of passing a guidewire through or near the center of the thrombus until the guidewire is beyond the thrombus, passing a delivery catheter over the guidewire across the thrombus and then exchanging the guidewire for a retriever. The delivery catheter is thereby withdrawn, exposing the retriever to the clot to enable capture after expansion of the retriever. This is followed by withdrawal of the clot-containing retriever device into an aspiration catheter positioned proximal to the clot of sufficient diameter to enable retrieval of the device with retention of the clot. One such retrieval device is the Solitaire™ revascularization device.
Most clot capturing elements of current retrievers are made of Nitinol and passively expand into and through the clot using the intrinsic radial force of the device prior to retraction of the device to remove the thrombus. This radial force is through thermally-induced, martensitic transformation of the Nitinol elements of the device by a predetermined shape change in the geometry of the Nitinol. By necessity, this expansion produces shearing and fracturing of the thrombus, since the clot capturing elements cannot expand through the clot to contact the vessel wall without producing linear defects within the clot. This clot shearing and fracturing can in turn increase the risk of distal embolization.
An estimated 700,000 patients in the U.S. and 950,000 in Europe suffer ischemic strokes each year, the majority of which are large vessel occlusions (LVOs). Some of the pivotal randomized prospective clinical trials to date have established the superiority of radially expandable retriever devices over drug therapy alone for LVOs of the anterior cerebral circulation. These clinical trials include: MR CLEAN, with published analysis appearing in the New England Journal of Medicine 2015; SWIFT PRIME, with published analysis appearing in the New England Journal of Medicine 2015; ESCAPE, with published analysis appearing in the New England Journal of Medicine 2015; EXTEND IA, with published analysis appearing in the New England Journal of Medicine 2015; REVASCAT, with published analysis appearing in the New England Journal of Medicine 2015 and THRACE, with published analysis appearing in Lancet Neurology 2016.
The published analyses of these trials have not emphasized the number of new ischemic strokes in the interventional arms of these studies compared to standard medical therapy. These new strokes are attributed to distal embolization, vessel spasm or dissection. In the MR CLEAN study, 5.6% (13 of 233) of patients in the retriever arm had clinical signs of a new ischemic stroke in a different vascular territory compared to 0.4% (1 of 267) in the control group, representing a 14-fold relative risk. In the REVASCAT study, 4.9% of patients had distal embolization into a new vascular territory compared to none in the control group, and 12.7% had local arterial complications attributed to passage of the retriever device (3.9% dissection, 4.9% perforation, and 3.9% vasospasm requiring treatment) compared to none in the control group. In the THRACE study, 6% had distal embolization in a new territory, and 26% experienced vasospasm, dissection or perforation. None of these events occurred in the control group.
The number of additional passes (withdrawal of the retriever across the initial clot-containing segment of the vessel) has also been underemphasized in the clinical trials of these devices to date. Each pass of a retriever requires manipulation and advancement of a catheter and guidewire across the segment of the thrombus-containing artery to enable repeat delivery/deployment of the retriever, thereby creating potential vessel injury in the form of vasospasm, dissection or perforation. In a recent study, as many as 5 separate retriever passes were required for intended thrombus removal (REVIVE 2018) with a mean of 2.2 passes. The Instructions for Use (IFU) for the Solitaire™ thrombectomy device (sold by Medtronic) instructs physicians to perform no more than three recovery attempts in the same vessel. The IFU for the Trevo™ thrombectomy device (sold by Stryker) instructs physicians to exercise caution when withdrawing the device through an area or arterial vasospasm. The 3D Revascularization device (sold by Penumbra) enables up to 5 passes of the retriever device, each of which requires re-crossing of the target vessel (e.g. M1 segment of the middle cerebral artery) with a catheter and guidewire.